In satellite retransmission of communication signals, two linearly polarized signals, rotated 90 degrees from each other, are used. Circularly polarized signals are also used. In less expensive installations for receiving linearly polarized signals, the feed horn for the receiving system is installed with orientation parallel to the desired signal polarization. The other polarization is not detected and is simply reflected back out of the feed horn. For more expensive installations, the entire feed horn and low noise amplifier ("LNA") system is mounted on a rotator similar to the type used on home television antennaes to select the desired signal polarization.
While the above-mentioned systems are cost effective, they are mechanically cumbersome and limit system performance. Other prior art signal polarization rotators electrically rotate the signal field in a ferrite media. While such rotators eliminate the mechanical clumsiness of the above-described rotators, they are expensive and introduce additional signal losses (approximate 0.2 DB) into the receiving system. See, for example, such an electronic antennae rotator marketed under the trade name "Luly Polarizer" by Robert A. Luly Associates, P. O. Box 2311, San Bernardino, CA.
The present invention eliminates the mechanical disadvantages of several prior art rotators and eliminates signal losses associated with other prior art rotators. A signal detector constructed according to the principles of the present invention comprises a transmission line having a signal received probe portion ("RP portion") mounted in a dielectric rod at one end of a circular waveguide and a signal launch probe portion ("LP portion") extending into a rectangular waveguide perpendicularly coupled to the circular waveguide. The RP portion of the transmission line detects polarized incoming signals in the circular waveguide and the LP portion launches the detected signal into the rectangular waveguide for transmission to an LNA.
In one embodiment of the present invention, the transmission line, by its coupling to the insulator rod, may be rotated continuously and selectively by a servo motor mounted on the waveguide assembly. As the RP portion rotates to receive the desired signal, the LP portion also rotates. However, the launched signal or the signal received at the LNA is unaffected because rotation of the LP portion is about its axis of symmetry in the rectangular waveguide. The RP portion in the circular waveguide rotates between the two orthogonally polarized signals impinging on the feed horn. By rotation to the desired polarization, that signal is received and the other reflected. The selected signal is then conducted along the transmission line to the rear wall of the circular waveguide portion of the feed horn and is launched into the rectangular waveguide by the LP portion.
Circularly polarized microwave signals are either left or right circular polarizations, LCP or RCP, respectively, and comprise two linearly polarized signals. Such signals are also used in earth-to-satellite communications to provide polarization diversity and frequency re-use. For convenient processing of such signals, it is desirable to transform them into linearly polarized signals.
In another embodiment of the present invention, a dielectric insert is interposed between the RP portion and the incoming signal. The insert transforms RCP or LCP into linearly polarized signals rotated 90 degrees from each other in accordance with well-known principles described in the prior art as the "delay-advance technique". In that technique an impedance is introduced into the transmission line to delay one component of the RCP and LCP. Since the RP portion can be rotated to any desired orientation in the circular waveguide, RCP or LCP can be conveniently selected and received by this feature of the present invention.